Polypropylene compositions have gained wide commercial acceptance and usage in numerous applications because of the relatively low cost of the polymers and the desirable properties they exhibit. In general, polypropylene polymers, particularly propylene homopolymers, have a disadvantage of being brittle with low impact resistance, especially at low temperatures. To combat these issues, manufacturers have incorporated rubber, which forms a dispersed phase within the polypropylene matrix. These two-phase materials are referred to as impact copolymers or ICPs.
While impact resistance is improved, a major drawback to such materials is the poor transparency, mostly due to the rubber particles being large enough to affect light transmission in the heterophasic system. Accordingly, several attempts have been made to improve the transparency of heterophasic polymer systems.
Some ICPs have been able to achieve clarity by refractive index matching the dispersed phase to the matrix material. Other ICPs have sought to achieve clarity by increasing dispersed phase miscibility to achieve particles which are not large enough to affect the light transmission. While these approaches may produce clear materials, they generally lack the stiffness and/or toughness of a conventional ICP. It would be desirable to have an ICP which is clear, stiff and tough, and which is not based on either technique.
ICPs can be formed by blending a rubber with the matrix phase, but it is more preferred, from a cost-to-produce standpoint, that they are prepared using an in-reactor process, where the matrix and the dispersed phase are formed in separate reactors, typically operated in series. Thus, it would be desirable to have an in-reactor ICP which is clear, stiff, tough and which is not based on refractive index matching or using rubber with increased miscibility with the matrix.
The present invention is directed to such a polypropylene impact copolymer. The ICP of the present invention is a composition comprising: (a) a matrix phase which comprises from 60 to 95 weight % of a polypropylene polymer containing from 0 to 6 mole % of units derived from one or more alpha-olefins other than propylene, and (b) a dispersed phase which comprises from 5 to 40 weight % of a copolymer derived from propylene and ethylene, where the units derived from ethylene comprises from 55 to 80 weight % of the dispersed phase. The ICP is further characterized by having a beta/alpha ratio less than or equal to 1.1.